App thinning automatically detects the user’s device type (i.e. model name) and only downloads relevant content for the specific device. In other words, if you’re using an iPad Mini 1 (which does not have a retina display but rather a 1x resolution), then only your 1x files (more on this in a moment) will be downloaded. The assets for more powerful and sharper iPads (such as the iPad Mini 3 or 4) will not be available for download). Because the user needs only download the content that targets his/her specific device, this speeds up the download process and saves space on the device.
There are three aspects of app thinning.
1. App Slicing: Slicing is the process of creating and delivering variants of the app bundle for different target devices. App Slicing delivers only relevant assets to each device (depending on screen resolution, architecture, etc.) In fact, app slicing handles the majority of the app thinning process.
When you’re ready to submit the app, you upload the.IPA or.App file to iTunes Connect, as you typically would (but must use Xcode 7 as it contains the iOS 9 SDK with support for app thinning). The App Store then slices the app, creating specific variants that are distributed to each device depending on its capabilities.
2. On Demand Resources : On-demand resources are files that can be downloaded after the app’s first installation. For example, specific levels of a game (and these levels’ associated content) could be downloaded only when the player has unlocked them. Further, earlier levels that the player has not engaged with after a certain set time can be removed to save storage on the device.
Enabling on demand resources involves changing the “Enable On Demand Resources” boolean to “Yes” in Xcode settings (under Build Settings).
3. Bitcode : Bitcode makes apps as fast and efficient as possible on whatever device they’re running. Bitcode automatically compiles the app for the most recent compiler and optimizes it for specific architectures. Can be turned on by the project settings under Build Settings and selecting bitcode to YES.
App thining process can be tested by TestFlight software and install in various devices. Size may vary.